To grow good arugula, wait to plant seeds until the weather is warm. Then your arugula will grow into nice big easy-to-harvest heads, instead of tall scraggly plants where you have to pick each leaf.
Most people who have grown arugula in the coastal Northwest think of it as a bushy plant that you harvest one leaf at a time. Tediously pick, pick, pick. They have never seen it as dense heads, like leaf lettuce, that you can cut all at once. The clue to this easy arugula is plant physiology.
Arugula is in the mustard family (Brassicaceae), which includes kale, collards, cabbage, and many others. Colloquially, they’re known as “brassicas”. A lot of brassicas “bolt”, much in the sense of a startled horse suddenly dashing away. Both the horse, and the vegetable crop, you’d prefer would stay there for your convenience. The plant does not physically go anywhere, but abruptly shoots up a stalk, blooms, and then dies. In any case, it’s gone. Or at least on the way out.
Typically, a brassica’s signal to bolt is a period of cold weather, in the range from freezing up to about 40°F (5°C). People often misread this. They see a brassica bolting in the spring, and they think “Oh, it’s the warm weather that made it bloom”. No. It’s the preceding cold period that induced it to bloom. Then, the blooming continues into warm weather.
Plants do this to make use of seasonal growth cycles. This is why a lot of brassicas are biennials. A cabbage, for example. You plant a cabbage start in your garden, and it grows all summer. The head just gets bigger and bigger, no hint of flowers. If you don’t harvest the cabbage, but let it go through the winter, next spring the head opens up, stretches out into long stalks, and becomes a mass of yellow flowers. Gardeners don’t usually see this because they harvest the cabbage to eat, but seed growers have to let it happen because that’s where new cabbage seeds come from.
In this case, the cabbage “uses” the winter cold signal to know when to bloom. Plant’s don’t keep a calendar, they can only detect conditions. The lack of bloom signal the first summer means the plant can grow big, and store up lots of strength. Then, next year when it does bloom, it can grow stalks tall, to scatter seeds far and wide.
“Classic” biennials, like the cabbage, typically die after they bloom, simply because they put all their substance into making seeds. But different brassicas vary this.
For example, garden bittercress, or shotweed, (Cardamine oligosperma) shortens the cycle. The seeds don’t germinate till the late summer rains. The plants grow through the winter, effectively behaving as winter annuals. In the spring, after winter chill has given them the signal to bloom, they bolt, shoot their seeds, and die in summer drought.
Perennial brassicas, such as sea kale (Crambe maritima) live from year to year, but again the cold period each winter serves as the signal to bloom.
Even classic biennials will occasionally behave as annuals, or as short lived perennials. If you plant a lot of, say, radishes, a few of them may bolt the first year, and so behave as annuals. On the other hand, if you have a biennial such as collards, and cut off all the flowers when it blooms, the plant may retrench. It may “forget” about blooming, put out side shoots, and grow for another summer. You may be able to get it to do this for three, four, or even more years, thus getting it to behave as a perennial.
The winter cold signal is still in operation, though. Radishes have more tendency to bolt if planted early, in spring, so they get a taste of the chill. Your “perennial” collards will still try to bloom each spring, after winter cold.
A friend told me a story of a garden on the big island of Hawaii. He saw a twenty-year-old collard plant that had grown, as he described it, “Like a great snake, along the ground.”. It makes sense. In that tropical climate, where it is never cold, the plant would never get the signal to bloom.
So now, back to arugula. I first grew arugula when I lived in Colorado. There, arugula was easy. It would self-sow. It typically behaved as a biennial. Overwintered plants would bolt in the spring, and then dry out and die in the hot summer. The stalks would blow around the garden like little tumbleweeds, scattering seed. Winter would come, too cold for seeds to germinate.
In the warmth of next spring, if there were good rains, seedlings would appear, and grow into nice, big heads, up to a foot across. All summer, there would be plenty of arugula, for salads, sandwiches, and even stir-fries. We used to make up a “bale” of washed arugula, packed tight into a produce bag, to take on hiking trips. In the cool of the mountains, it would ride well in a backpack. After a couple days of camp meals, the fresh greens were very welcome.
This bounty would last well into the fall. Finally, usually in November, temperatures in the single digits would ruin the arugula. This wouldn’t kill the plants, but frost-burn the leaves, so you had to fuss with it too much to get anything edible.
Next spring, those overwintered plants would bolt, and scatter their seeds to complete the cycle. It was so easy. The arugula would grow itself. The main work was pulling out the extras. Naturally, I didn’t pay much attention. There was always plenty of arugula.
But one year, nobody had any good arugula. As soon as the plants appeared, it seemed, they would bolt, putting up their stalks of pale yellow flowers. They had slim, scrawny stalks, with only a few small leaves. This is when I started to figure out what was going on.
Colorado is a capricious climate, different every year. Often, the weather would go from cold to hot, with little in between. But in the Year of Scant Arugula, there was a long, wet, chilly spring.
What happened was this. The spring weather was warm enough to germinate the overwintered arugula seeds. But then, the weeks of chilly weather gave them their cold requirement. So, the plants all bolted and went to seed. They acted as annuals. They finished up when hot summer came. And no arugula to eat.
When I moved to Portland, I was appalled at the miserable arugula here. A lot of people have some arugula in their gardens, which they have long since disregarded. They may not even remember it is arugula. They planted some long ago, and it was not very satisfactory, but they never entirely weeded it out. So it has been self-sowing ever since.
Seeds ripen in the summer, and get scattered around. In this maritime climate, the typical cycle is this: Most seeds germinate in the first autumn rains. The plants grow all winter. The cold is seldom enough to even nip their leaves. However, in our long, chilly, rainy fall, the plants become thoroughly convinced it’s time to bloom. It’s seldom even a bolt, more of a vertical stalk from the get-go.
If they get started early enough, they may be blooming by late fall. If the weather gets too wintry, they may bide their time till spring. Any seeds that didn’t sprout in the fall will do so in the first warmth of spring. But the weather stays cool; and, here too, these plants are convinced to bloom.
So, Northwest arugula tends to exists as tall, stalky plants. In rich garden soil they may be branching bushy things a couple feet tall.
In poor conditions, it may be not much more than a single stem.
In a site that dries out in the heat of summer, plants may die after making seeds. And so behave as annuals. In a cushy garden bed, with moisture all summer, plants my make it through till autumn rains re-invigorate them. And so behave as perennial.
In either case, though, it’s awkward to make use of the greens. There are two easy ways to harvest greens; the “shear” method and the “pluck” method.
For the “shear” style, you plant the seeds thickly, so the greens grow up all packed together. To harvest, you slice the leaves off at the base, much like mowing a lawn. If all goes well, the plants will push up new leaves, for more harvests later.
To “pluck”, you start seeds further apart, so there are distinct plants. As they grow, and begin to touch, you thin them out. You usually take the biggest ones first, leaving the smaller ones to grow and fill in.
Neither of these work very well with stalky arugula. If you try to shear a thick stand like this, you get a handful of wiry stems. They don’t tend to grow back very well, because they have been putting most of their strength into flowering. And if they do re-grow some, it’s just more stems. If you pluck out individual plants, again, you have a lot of tough stem. You have to pick the leaves off individually to use.
After all this discouragement, I tell you, it doesn’t have to be this way. The trick is to gather the seeds, and keep them from sprouting until you want them to. The easiest time is mid- to late summer, when the mature arugula plants will be covered in drying pods. Take a big bowl, or paper bag, into your garden. Hold the seed stalks over, and gently rub or crush the tops. As the pods shatter, catch the seeds. You will get lots of pod halves too, but the seeds will be little brown spheres about the size of a millet grain. You can sift out only the seeds if you want, but there is no need. Just keep the seeds dry through fall and winter. Next spring, wait until the weather is warm before planting them, temperatures not dropping into the 40’s Fahrenheit.
Since most Northwest gardeners have never seen how this works, I decided to do a demonstration. I planted a patch of arugula from seed on July 6. I put it on drip irrigation, because by that time of summer it’s hot and dry. I could have planted it much earlier, but it demonstrates that you can grow arugula like this, even when summer is well along.
Arugula is very easy to start from seed. Just scatter, roughen the ground, and water a few times. Like chia, arugula seeds have a coating that absorbs water and swells up to form a drop of jelly around each seed. This holds moisture, to aid germination.
The seeds I planated popped up right away, and I started using the thinnings. The next time I thought to photograph them was in about six weeks, on August 19. As you can see, they are compact rosettes; no flower stalks.
For harvest, I personally prefer the pluck method. I say, “Clean kill”. I guilt-trip people by comparing it to hunting. You wouldn’t trap an animal, hack off one leg, and then let the poor thing run around bleeding. By the same token, why hack off individual leaves? The plants are usually too crowded anyway. Respect life, and take the whole thing. Clean kill.
This method is particularly easy with arugula, as each plant has a single taproot. When the plants were small, I would just pull them out.
When they got bigger, I would snip the root at ground level, and pop out whole heads.
A very few of these plants had a few flowers. But, unlike cold-sewn arugula, where the whole plant becomes a flowering bush, these were insignificant.
I like arugula because, of all brassica crops, it is resistant to white fly. I don’t know how it manages this. There is a particular species of white fly, the cabbage white fly (Aleyrodes proletella) that loves brassicas. I have had other brassica crops, such as collards and Brussels sprouts, nearly eaten up by these white flies. Clouds of them would arise when ever I brushed by the plants. The leaves would be curled and distorted, and black with mildew from the white fly honeydew.
Arugula is not immune, but somehow keeps white fly in check. While other brassicas were overwhelmed by the flies, the arugula had only a few. In the picture below, there is a small colony of white fly larvae, but barely noticeable.
Magnification proves they really are white fly. There they are, but not a big problem
Some people are horrified at the idea of growing arugula in hot weather. They are sure it will be too peppery, and tough. I have to say, I can’t tell the difference. Arugula is always peppery and hot. If you don’t like that, you don’t like arugula. Try it and see what you think. If it’s too spicy, you can chop it fine and stir-fry it. Cooking takes away all the hotness of brassicas.